That sounds like a systemic reaction. Have you had a detailed blood draw done? One thing I would look at is your thyroid. It may sound weird, but I experienced some systemic reaction prior to getting diagnosed with hypothyroidism and started taking medication to correct it. It also caused flare ups within my nerves. So my sensitivity to pain and itching was increased on my skin in several places. I could not take hot showers at all. Started out just red blotchiness and slight swelling, then started to burn my skin all together. Took about 6 months to fully regulate.
Another possibility, have you ever had chicken pox? It could be a variation of the shingles virus that just hasn't gone full on yet. My mom had blotches pop up that were burning and itching, but not too bad until the join pain started. Then her blotches got worse. But it took it's time with her.
I hope you find some answer before anything changes or gets worse.